Electron source



P 1952 H. w. WASHBuRN 1 ELECTRON SQURCE Filed-Oct/l, n48

an AMPL/F/ER IN VEN TOR. HAROLD m mas/mum gwww A T TOR/VE Y PatentedSept. 23, 1952 ELECTRON SOURCE Harold W. Washburn, Pasadena, Calif.,assignor to Consolidated Engineering Corporation, Pasadena, Calif., acorporation of California Application October 1, 1948, Serial No. 52,341

4 Claims. (01. 250-419) I This invention relates to mass spectrometersin general and in particular to improvements therein relating to theformation of an electron beam for the ionization of sample molecule inthe mass spectrometer.

A mass spectrometer is an analytical apparatus which functions to sortand measure ions. Ordinarily it includes an ionization chamber intowhich molecule of the sample to be analyzed are introduced. In theionization chamber the molecules are bombarded by a stream of electronsand converted into ions. So called Dropelling or accelerating electrodespropel the ions from the ionization chamber into and through an analyzerchamber.

During passage through the analyzer chamber the ions are subjected to atransverse electric or magnetic field or both, to separate themaccording to their mass-to-charge ratios into a plurality of divergingbeams of ions. Each ion beam is composed of ions of the same specificmass and differing from the mass in the other beams. The diverging beamsare successively focused on an ion collector by varying the potentialapplied to the accelerating electrodes or by varying the intensity ofthe transverse field in the analyzer tube. The current produced by thedischarge of each beam on the ion collector is a measure of the partialpressure of the molecules (from which the ions were derived) in thesample being analyzed.

In one type of mass spectrometer, ions are formed within the ionizationchamber by an electron beam directed through the chamber at right anglesto the path of the ions. A conventional instrument is equipped with aribbonfilament for the production of electrons which are directedthrough a slit in the wall of the ionization chamber transversely acrossthe chamber and through a slit in the opposite wall to impinge on anelectron collector or target. In this arrangement the ribbon produces anelectron beam with a sufiiciently large cross section that warpage ormovement of the filament still permits the size of the cross section ofthe beam to be determined by the slit, commonly referred to as theelectron slit. The filament is made in the form of a thin ribbon so thatan electron beam of large cross section is produced with low end effectsby a reasonable length of filament. The thin ribbon also gives areasonably low power consumption.

I have now found that another possible arrangement is to use a filamentof sufficiently small cross section so that the thickness of theelectron beam is determinedby the size of the filament rather than bythe size of 'the'electron slit. This arrangement has the followinggadvantages over the conventional electron..,gun,' described above: i

1. For the same emissiondensity and the, same cross section of theelectron beam, the watts consumed by the small filamentare less due tothe small radiation area; I y

2. The filament can be made sufficientlysmall that reasonable wattage.consumption canbeyobrtained when using a filament with a circular crosssection. In this respect the circular cross section has the advantage ofhaving a longer life.

The reason for the longer life of a:"fi lament with a circular crosssection may be appreciated from the following comparison. For example,comparing the life of an 8 mil diameter filament. with the life of a 1by 40 mil fiat ribbon filament, considering that the life is determinedby evapoe ration, it will be seen that the roundfilamentwill have a lifecorresponding to at least ,oneymil evaporation of tungsten. Onthe'otherhand, it would probably be necessary to discontinuert use of the fiatribbon filament after 0.1 mil had evaporated.

There is one problem which becomes.,mo re. acute, but for which I haveprovided asimple solution when the physical size of the filament; ratherthan thesize of the electron slitis used to limit the thickness of theelectron beam. When the thickness of the beam isto be limitedby the.size of the filament and where the filament is a magnetic field of lowmagnitude, say .500 gauss. or less, it is necessary toprovide somesortof. electric field focusing. 1

There are many means of obtaining this elece tric field focusing but apreferred method inac-i cordance with the invention is to mount ashield, either flat or cupped, on the side of the filament opposite theelectron slit. By adjusting the voltage on the shield it is possible to.driveiall-of the electrons, emitted from the-filament,;back-' into thefilament except for a narrow strip on the electron slit side of thefilament. At the same time it is possible to produce an electrical fieldon the emitting side of the filament which is substantially parallel tothe desired direction of motion of the electrons or I may produce afield which is slightly converging so that any side. energies impartedto the electrons are in such a' direction that thethickness of theelectron beam is reduced. This latter feature is highly important sinceany side energies which will thicken the beam and make it larger thanthe filament will impair the resolution of the mass spectrometer.

In accordance with the invention therefore, I provide in combinationwith a mass spectrometer having an ion source comprising an ionizationchamber, a slit in a wall thereof for introducing electrons into thechamber and a sample inlet tube, the improvement comprising an electronemittingfilament of small cross section, preferably circular, mountedadjacent the ionization chamber and aligned with the slit, and afocusing electrode disposed on the side of the filament opposite theionization chamber and being at a potential such as to drive theelectrons emitted from the filament back into the filament except for anarrow strip on the ionization chamber side of the filament. I

As pointed out above, this arangement serves to determine the dimensionsof the electron beam and the size of the slit through the wall of theionization chamber is no longer critical. The invention will be betterunderstood from the following detailed description thereof taken inconjunction with the accompanying drawing in which:

Fig. l is: a diagrammatic. view of a conventional 180 mass spectrometershowing the ionization chamber: end of the instrument blown up toapproximately three times its comparative size;

' Fig. 2 isa line diagram of the electric. field produced'by thefilament shield and the electron slit when the voltages are properlyadjusted.

Fig. 3 is a sectional elevation of an ion source as described byClifiord E. Berry in United States patent application Serial No. 33,596filed June 17, 1948, showing means of employing the electron source ofthe invention in conjunction therewith;

Fig. 4 is a section taken on the line 4-4 of Fig. 3; and

Fig". 5 is a perspective view of the electron source'of the inventionmounted as described by Gordon'W'. Pallette in United States patentapplication Serial No. 40,481, filed July 24, 1948. In the drawings theinvention is shown in association with a 180 mass spectrometer of thetype wherein propelling electrodes are provided in the. ion source. Aswill be apparent from. the following description, the invention isequally adapted to incorporation in substantially any mass spectrometerwherein the sample to be analyzed'is ionized by means of an electronbeam. The invention is independent of the particular focusing means, thepath of ion travel or the use-of accelerating electrodes.

Referring to Fig. 1, it will be observed, that it shows a massspectrometer having an ion source ll, an analyzer tube I2, and an ioncollector l3 disposed within an envelope M which is'kept at lowpressures during the operation of the instrument. The analyzer tube I2is provided at the end adjacent the ion collector l3 with an exit slit Ithrough which the ion beams are focused onto the collector IS.

The spectrometer is provided with a pumping system or envelope exhaustline [6 which may be connected with a mercury diffusion pump, molecularpump, or any appropriate evacuating system not shown. The analyzer tubei2 may be provided with ports 11, by means of which the ion. source andthe analyzer tube are evacuated through. the envelope. Alternatively,the envelope may be ommitted by making the analyzer tube l2 gas tightand, attaching the pumps to it by means which are well known. An inletline l8 provides means for introducing a sample to be analyzed. Anelectron gun 20 in accordance with the invention is mounted adjacent toor as part of the ion source and cooperates with an electron target orcatcher 21 to discharge a beam of electrons through an electron slit 22in a wall of the ion source adjacent the gun 20 and a second electronslit 23 in an opposite Wall of the source adjacent the collector 2!. Apropelling electrode 25 and accelerating electrodes 26, 2'! are disposedwithin the ion source so as to propel the ions formed therein into andthrough the analyzer tube. Representation of this particular ion sourceis for illustrative purposes only, there being many modificationsthereof, equally applicable to this invention.

As above described, the ions propelled from the source are formed in theanalyzer tube into diverging beams of ions of given specific mass. Thesediverging beams are successively focused on the ion collector l3 throughthe, exit slit [5 by varying the potential applied to the acceleratingelectrodes 26, 21. The ion collector i3 is linked with an amplificationand sensing circuit by a lead 29 sealed through the wall of the envelopel3. The lead 28 connects the collector 20 to an amplifier 30 which is inturn linked to a sensing means (not shown)v such as av recorder or thelike.

Accelerating electrodes 26, 21 are connected through leads 32, 33respectively to a conventional voltage supply circuit. In a similarmanner the propelling electrode 25 and the electron gun 20 and catcher2| are connected to a more or less conventional voltage supply circuit.

The electron gun 20 shown in Fig. 1 comprises a filament 34 preferablyof circular cross section and disposed adjacent the first electron slit22. An electron shield 36 is disposed adjacent the filament 34 on theside oppositethe electron slit 22. A focusing electrode 31' is disposedbetween the filament 34 and the slit 22 and has a slit 31A therein inalignment with the filament and electron slit 22. By establishing theproper potential between the shield 36, the filament 34, and thefocusing electrode 31, the electrons emitted from the filament may bedriven back into the filament except for a very narrow beam emanatingfrom the filament from the side adjacent the focusing shield. In thismanner, and by making the slit width greater than the filament diameter,the electron beam width becomes independent of the dimensions of theelectron slits 22 and 31A.

The electric field established around the filament 34 is illustrated inthe line diagram of Fig. 2 showing the filament 34, the focusingelectrode 31 and the shield 36. Representative potentials are shown; theshield being at .50 volts, the filament at 0 volts and the focusingelectrode at +20 volts. In the drawing of Fig. 2 the shield 36 is shownas a fiat surface rather than in the arcuate form shown in Fig. l. Theshield electrode 36 may be either flat or arcuate, the fiat form beingshown in the diagram of Fig. 2 to simplify the illustration of the fieldforces set up thereby.

Referring to Fig. 2 it is seen that a portion of the electrons emittedfrom the filament 34 are driven back into the filament. The phenomena isillustrated by lines 40, 4|, 42 etc. Another portion of the electronsemitted from the filament 34 are propelled in the direction of thefocusing electrode 31 and converge towards each other. The electricfield controlling the motion ofthe electrons is represented by the lines43, 44. and 45.

Ii Theillustration of Fig. l is: diagrammatic only in that it does notshow specific means for mounting the electron source of the inventionwith respect to a specific ionization chamber. In Fig. 3 I have shown insectional elevation anion :source ofthe type described by Clifford E.Berry in United States patent application Serial No. 33,596,

'filedJune 17, 1948. This particular ion source is chosen as a means ofillustrating the application of the electron gun of the invention to anion source but is not intended to limit the invention in its applicationto any particular source. As described above, any ion source willcomprise a substantially enclosed chambenkcommonly re ferred. to as anionization chamber, having an inlet'and an outlet and atleast oneelectrode for discharging ions from. the chamber. after they have beenformed therein. The electron source of the invention is adapted to beemployed with any ion source of this nature. In Fig. 5 the electronsource is shown. in a perspective view as mounted in accordance withmounting means described by Gordon W. Pallette in co-pending respect tothe ionization chamber and means'of applying differing potentialsthereto is described in detail in the aforesaid co-pending applicationof Clifford E. Berry and forms no part of the present invention.

As shown in Fig. 4 the body of the ion source 60 is generallycylindrical in section. A portion of one wall of the source is cut awayto form a fiat face extending from the lower end of the ion source (Fig.3) to a point substantially in lateral alignment with the outlet end ofthe ionization chamber 6|. An arcuate shield 12 forms with the face 10an enclosure H open at itslower end and closed at its upper end by anoverhanging lip 13.

The electron source itself (see Figs. 3, 4 and 5) comprises a filamentor wire 16 preferably of circular cross section attached at oppositeends to terminal posts Ti, 18. Preferably the terminal posts are sprunginwardly a small amount when the wire is afiixed'to their top end so asto produce an expanding tension on the filament. The terminal posts aresealed through an insulating base 80 and project through the oppositeside of the base for connection to suitable electric leads (not shown).A post 82 is sealed in the base 80 intermediate the terminal posts I1,18, and supports at its upper end (see Fig. 5) a positioning member 84.The positioning member is shaped to fit within a receptacle 86 formedcentrally on the flat face 10 of the ion source body. As described inthe aforementioned co-pending application of Gordon W. Pallette, byfitting the positioning member within the receptacle and affixing thesame thereto the position of the filament is fixed with respect to afirst electron slit 94 in a wall of the ion source. A diametricallyopposite slit 95 opens into a cavity 96 in which is disposed an electroncatcher 98.

6 As shown in Figs. 4 and 5 and as described by Pallette, the posts 82upon which the positioning member is mountedisdisplaced forwardly fromthe plane intercepting'the terminalposts-H, .18. This permits insertionof the positioning member in the receptacle 96 and proper spacing; ofthe filament 16 with respect to its lateral displacement from the wallof the ionsource. Inaccordance with the. invention the mounting means isprovided with a pair of auxiliary conductive posts I00, Hll sealedthrough the base and extending parallel to the terminal posts H, 18andgin-termediate these posts and the center post 82. A semi-circularelectrode 104 is mounted-to the upper end of the posts 100, NI soas toexten partially around the filament l6. I In this embodiment of theinventionthe electrical shaping field around the filament 1,6. is formedby the electrode I04 and by thewall of the ion source 60. The focusingelectrodei, e. the electrode 31 (Fig. 1) is omitted. The useofadditional focusing electrode 1 t optionalfeature of the invention sincethe; ions ource itself may be established at a potential such as toaccomplish the purpose of the focusing electrode. As above describedthe. electrode 36 (Fig.4) or, the electrode 104 (Figs. 3, 4 and 5) maybe eitherarcuate in section as shown in Figs.-;l, 3, 4 and 5 or it maycomprise a fiat plate as. illustrated schematically in Fig. 2. Referringtothe electron source shown in Fig. 5it will be necessary, if using afiat electrode to replace the electrode 104, to offset the conductiveposts 100, I9! rearwardly as viewed in Fig. 5. This may be easilydone soas to disposelthelfiat electrode in a planelying behind the filament l6adistance approximately equal to the spacing between the filament nd thearcuate electrode I04. By using an electron source of the typedescribed, regardless of the means by which it is mounted to the ionsource or regardless of the type of ion source employed, the dimensionsof the first electron slit 94 communicating between the electronemitting filament and the inside of the ionization chamber are no longercritical. I-Ieretofore the shaping of the electron beam has beendetermined by the dimensions of this slit Which were therefore highlycritical.

The invention has been illustrated and described with reference to theuse of a filament of circular cross section. Although use of such afilament constitutes preferred practice, filaments of irregular sectionor comparatively narrow ribbon filaments are satisfactory. In any case,filament dimensions should be selected, such that the size of theelectron beam will be determined by the filament in association with theshaping field irrespective of the size of the electron slit in theionization chamber wall.

One feature of the Pallette mounting means which makes it particularlyadvantageous for use with the electron source of the instant inventionis the accuracy with which its positioning may be fixed. Since thecharacter of the electron beam is determined in my electron source bythe size of the filament and by the shaping field produced between theshield electrode and the ion source or auxiliary focusing electrode, thepositioning of these elements with respect to the ionization chamber orthe slit opening thereinto is important. Although Pallette'shows oneexcellent means of establishing a fixed relationship between theelectron emitting filament and the ion source as such, any such meansare within the contemplation of the invention.

Rslitiiini a iawall'fthe'reof for: admitting: a beam of selectrons xintothe vchambenan electron: emitting filament bfismaller crossisection'than theminor we axis of the slit bpeni'ng mounted adjacent theiiionizati'om chamber and "in alignment with the slit: and means forproducing aishaping electrical lifield around'i the filament toform" theelectrons -='emitted from the fiiament into an electronbeam of smallercross seetion than the 511mm directed throughthe slit.

' --"2; -Ina 'mass 'spectrometer ion' source for use in a relativelyweak magnetic-'fieldythe combina- "tion comprising an ionizationchamberhaving a @slit' inswan thereoffor admitting abeam ofreieetrons'in'to the chamber, arr-electron emitting filament or'cireular'- cross section and; of smaller.

diameter than 'the-minor axisof'the slit opening "mountew adjacent" theionization chamber" and in alignm'ent' with the" slit; a:semi-cylindrical electrode mounted coaxialiy with "the filament"andorftheside thereoi opposite the ionization i chamber; and means forimpressing a potential "-"ori' the electrode to direct the electronsemitted by the" filament througntheslit' as a' beam of smaller *crosssection than the slitopening.

3'; "In-amassspectrometerion source for use in a'relatit'elywe'akmagnetic field; thecombinatioircomprising tan-ionization chan'iberhaving a slit? in the -wa1l"thereof= for admitting a beam'mf*eleactronsintd-thechamber, anelectroni'emitting -"filam'ent'of smallercross s'ectiomthan'the minor 0 taxisfoi" the" slit opening mountedadjacent the chamber and 'in" alignment with .the "slit," "and meansforv producing af'shaping electrical" field iaround the filament i r toi-.'form the r electrons 1 emittedrfromthe filament into anelectron beam1 :of smaller cross section than the slit :and directed .throughrtherslit.

* 4; In a mass. spectrometer ionsourceclforuse 1 inc; relatively weak'magnetic field, the combina- .uslit; opening mo'unted adjacent the:ionization chamber; and in alignment with: the 's1it,na first..zelectrode.spaced 'from the filament: Eon the side opposite theionization -ohamber, .a second reiecl5- 'tionxcomprising an ionizationchamber: having a t slit in a2wall thereof for'admitting electrons intothe :chamber; an electron emitting filament of 'smallericrossfsectionthanathev minor axisiof the trade rdisposed between thefilament' vandthe t: ionization chamber and; having a slit thereini-in alignm'entswiththe slit in the'ionizationrchamriser and defining an opening having aminor axis 7 longer; .thanfthe cross 1 section 20f the electron 20emitting. filament, and rmeans for: 'impr'essing-:.a potential betweenthe" first and second-electrodes toxsha'pe the electrons:emittedi fromthe-filament into: a beam directed through the slits: in the 9 secondelectrode and .in thezioniza-tion chamber 25 and :of .a widthindependent oi. the dimensions of the slits.

.. 2 HAROLD WASHBURN.

--REFERENCES CITED Thefollowing references are of record in the'UNITEDTSTATES" PATENTS

